1. Field of the Invention
Embodiments of the present invention relate to techniques for performing reliability testing on computer systems. More specifically, embodiments of the present invention relate to a method and an apparatus for performing high-accuracy fan-speed sensing and control to assure vibrational integrity in a computer system.
2. Related Art
Computer systems such as servers and storage arrays can be adversely affected by mechanical vibrations of system internal components and structures. These vibrational problems are becoming more significant because of the following trends: (1) cooling fans are becoming increasingly more powerful; (2) chassis and support structures are becoming weaker because of design modifications to reduce cost and weight; and (3) internal disk drives, power supplies, and other system components are becoming more sensitive to vibration-induced degradation. For example, hard disk drives (HDDs) are becoming more sensitive to vibration because the storage density for HDDs has increased to the point where a write head has to align with a track which is less than 20 nanometers wide. Moreover, the write head floats only 7 nanometers above the disk surface. These extremely small dimensions make the read and write performance of the HDDs very sensitive to vibrations.
Some servers and storage arrays are equipped with a large number of cooling fans (e.g., twelve or more) that operate at very high speeds. Hence, cooling-fan-induced vibrations are a primary source of mechanical vibration in a computer system. Ideally, all cooling fans in a computer system operate at a constant speed which is determined based on design criteria. In practice, however, fan speeds in computer systems are not actively controlled, and hence the speed of each fan typically varies from the designed speed. As a result, multiple fans inside a computer system can produce a spread of mean fan speeds.
One problem associated with fan speed variation is that it can excite vibrational resonances within a computer system's mechanical structure. Specifically, if a fan's operating speed or an associated harmonic coincides with an internal vibrational resonance of the computer system, there can be a significant resonance-related amplification of the vibration which can cause system components such as disk drives and power supplies to fail. Moreover, when there is a spread in the mean fan speeds for multiple fans operating in parallel, the spread in mean fan speeds can cause a “beat” effect which creates numerous new frequencies. These new frequencies create additional opportunities for their harmonics to intersect with the internal structural resonances. Hence, it is highly desirable to be able to ensure that all fans operate at a desired speed.
Fan speeds can be measured by determining fan shaft rotation using a tachometer. Unfortunately, tachometer measurements of shaft rotations per minute (RPM) generally have a high degree of uncertainty. For example, in some server systems, the standard deviation of fan speeds is on the order of 7%. This measurement uncertainty can even exceed the differences between mean fan speeds in systems with multiple fans, making it extremely difficult to monitor fan speeds and to align them with a desired fan speed. Moreover, because each of the multiple fans is integrated with a dedicated tachometer, all these tachometers continuously produce a large quantity of fan speed data which can consume computational resources.
Hence, what is needed is a method and an apparatus that facilitates measuring and controlling fan speeds with high accuracy.